Declarative knowledge or descriptive knowledge, also or propositional knowledge, is the species of knowledge that is, by its very nature, expressed in declarative sentences or indicative propositions. This distinguishes descriptive knowledge from what is commonly known as "know-how", or procedural knowledge, that is, the knowledge of how, and especially how best, to perform some task.

What is the difference between knowledge and beliefs? A belief is an internal thought or memory which exists in one's mind. Most people accept that for a belief to be knowledge it must be, at least, true and justified. The Gettier problem in philosophy is the question of whether there are any other requirements before a belief can be accepted as knowledge.

The article Knowledge (philosophy) discusses the view of philosophers on how one can tell which beliefs constitute actual knowledge.

Types of knowledge

Knowledge can be classified into a priori knowledge, which is obtained without needing to observe the world, and a posteriori or empirical knowledge, which is only obtained after observing the world or interacting with it in some way.

Inferential knowledge is based on reasoning from facts or from other inferential knowledge such as a theory. Such knowledge may or may not be verifiable by observation or testing. For example, all knowledge of the atom is inferential knowledge. The distinction between factual knowledge and inferential knowledge has been explored by the discipline of general semantics.

Knowledge in various disciplines

There are many different disciplines that generate beliefs that can be regarded as knowledge. They include science (which generates scientific theories), law (which generates verdicts), history (which generates history), mathematics (which generates proofs) and theology (which generates truth).

Knowledge in science and engineering

Scientists attempt to gain knowledge through the scientific method. In this method, scientists start by finding a phenomenon of interest, which generates questions. A scientist then picks a question of interest, and based on previous knowledge, develops a hypothesis. The scientist then designs a controlled experiment which will allow her to test the hypothesis against the real world. She then makes predictions about the outcome of the test, based on the hypothesis.

At this point the scientist carries out the experiment, and compares her predictions with her observations. Assuming that there were no flaws in the experiment, then if they match, then this is evidence in favour of the hypothesis. If they do not match, then the hypothesis has been falsified. The next steps are peer review and publication, through which the results are distributed to other scientists.

A hypothesis that has been shown to accurately and reliably predict and characterize some physical phenomenon, and has been sufficiently peer-reviewed and tested, may become a scientific theory. Scientific theories are widely regarded as knowledge, though they are always subject to further revision or review should new data come to light.

To use scientific theories, they must be applied to the specific situation in hand. For example, a civil engineer might use the theory of statics (a branch of physics) to determine whether a bridge will hold up. This is one case where new knowledge is generated from scientific knowledge by specialising it to an individual instance.

There are a number of problems that arise when defining knowledge or truth, including issues with objectivity, adequacy and limits to justification. Beliefs are also very problematic not least because they are either true or false, and therefore cannot be adequately described by conventional logic. An action likewise can be taken or not, but there is the troubling idea of an "event" is, an action taken by nobody, or nobody who you can blame.

Non-scientific methods

Some people hold that science does not actually tell us about the physical world that they live. They hold that the world cannot be understood by science, but rather by religious revelations, mystical experience, or literary deconstructionism.

Practical limits for obtaining knowledge

What we hold to be knowledge is often derived by a combination of reason from either traditional, authoritative, or scientific sources. Many times such knowledge is not verifiable; sometimes the process of testing is prohibitively dangerous or expensive. For instance, some physics theories about the nature of the universe, such as string-theory, require the construction of testing equipment currently beyond our technology. Since such theories are in principle subject to verification or refutation, they are scientific; since they are not proven experimentally, they are not considered certain knowledge. Rather, in such cases we have certain knowledge only of the theory, but not of what the theory describes.

"Of the three ways in which men think that they acquire knowledge of things—authority, reasoning, and experience—only the last is effective and able to bring peace to the intellect." (Roger Bacon, English alchemist and philosopher)